The invention relates to a continuous process for the dehydrogenation of paraffinic hydrocarbons, most frequently those hydrocarbons which are gaseous at ambient temperature, comprising at least 2 carbon atoms per molecule, into corresponding olefinic hydrocarbons.
The present invention concerns, more particularly, the synthesis of isobutene by the dehydrogenation of isobutane. Isobutene is used, in particular, fop the preparation of MTBE (methyl tertio-butyl ether) for the purpose of improving the octane index of gasolines.
The process of the present invention comprises means for injecting sulphur into the charge to be treated and means for stripping the sulphur which is retained on the catalyst; these conditions make it possible for the stability of the catalyst to be improved and also its life.
There is substantial interest in the implementation of dehydrogenation processes with hydrocarbons, such processes being effective, selective and economical while also contributing to the formation of hydrogen, a product which tends to be lacking in current refining operations. This interest is especially justified by the valorization of saturated hydrocarbon cuts, in particular, of low boiling saturated aliphatic hydrocarbons, such as ethane, propane, butane, isobutane, pentane and isopentane, which are recoverable, in particular, after the removal of unsaturated substances from C3, C4 and C5 cuts which have been obtained by steam-cracking or catalytic cracking, and also from liquefied petroleum gases, often called LPG, or from field gases.
The reaction for the production of olefinic aliphatic hydrocarbons, often called alkenes by chemists, from saturated aliphatic hydrocarbons, often called alkanes by chemists, has been known for a very long time. This reaction has been described, in particular in the patents U.S. Pat. Nos. 3,126,426, 3,531,543, 3,978,150, 4,381,417, 4,381,418, and U.S. Pat. No. 4,704,497. It can employ a metal catalyst supported on a platinum base, a chromium oxide on alumina or a crystalline zeolitic catalyst with a silica and alumina base of the MFI type.
One of the problems to be resolved concerns the coking content of the reactor materials, lines and inter-reactor exchangers; in fact, under the reaction conditions (increased temperature and presence of olefins) the customary materials catalyze the dehydrogenation reactions and cause relatively fast coking of the metal surface.
This fact is particularly critical in instances where the operation is carried out in a regenerative system, that is to say continuously, for in these instances it is not possible to interrupt the functioning of the reactors to mechanically decoke them or to burn the coke, without changing the nature of the process altogether.